(a-2b)(a²+2ab+4b²)-a(a-5b)(a+3b)=(a-2b)(a²+2ab+4b²)-a(a²-2ab-15b²)=a³+2a²b+4ab²-2a²b-4ab²-8b³-a³+2a²b+15ab²=2a²b+15ab
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